634 



M O L L U S C A. 



M 'iiusca. nera, all these valves are so united, that the lines of se- 

 " V*' paration are not perceptible, while in others the double 

 lateral valves only are incorporated. Each valve con- 

 sists of an elevated and depressed portion. The elevat- 

 ed portion is conical with its base at the adhering part 

 of the shell, while the depressed part of the same form, 

 lias the base at the mouth. The former consists of co- 

 nical vertical tubuli, while the latter appears solid. 

 When the base is testaceous, it is either solid, or con- 

 sists of horizontal tubuli, radiating from a centre united 

 by a simple layer, exhibiting concentric circles. 



The structure of the valves gives sufficient indica- 

 tions of the manner in which they are formed. M. Du- 

 f'resne, in a paper published in the Annales du Muse* 

 urn I. p. 465, advanced the singular opinion, that the 

 nnimals quitted their old shells when they became too 

 small, and formed new ones suited to their size. The 

 arguments by which it is supported, indicate an igno- 

 rance of the structure of the shell, and the relations of 

 the parts of which it consists. To us it appears plain, 

 that each valve is increased in two directions, the ele- 

 vated part by an extension of the tubuli at the base, 

 and the depressed part, by the application of fresh mat- 

 ter to the side. The stria;, which are the indications 

 of successive depositions of matter, and the structure 

 of the valves themselves, point out this mode of en- 

 largement as the only one which can take place, even 

 on the supposition that the shell is frequently renewed. 

 By the growth of the elevated parts, the shell increases 

 in height and diameter at the base, while the growth 

 of the depressed parts, preserves to the rnouth, suitable 

 dimensions for the corresponding increase of size in 

 the parts of the operculum. It is obvious that this 

 increase of diameter at the base must be accompanied 

 by a corresponding enlargement of its covering. This 

 takes place by the extension of the horizontal tubuli, 

 and each enlargement is marked by a concentric ridge. 



This opinion which we have espoused, and which we 

 find hinted at by Lamarck, in his " Histoire Naturelle 

 dcs Animaitx sans Vertebres," v. p. 389, is founded on the 

 structure of the different valves, the indications of the 

 layers of growth, and the manner in which the valves 

 are separable from each other and from the base. We 

 might have entertained some suspicions of error, had 

 we not attended to the morbid appearances of the shell, 

 the restraints imposed on its growth by the situation in 

 which it lives, but especially the manner in which 

 fractures are healed, and abstracted parts restored by 

 the secretion of new matter. Such observations have 

 removed all suspicion, and demonstrated the truth of 

 the, explanation offered. It may be added, however, 

 that in the case of the inversely conical shells, the in- 

 crease probably takes place at the mouth. 



The mouth of the shell is closed by the cloak of the 

 animal, leaving in the centre a tubular or linear aper- 

 ture for the protrusion of the feet and entrance of the 

 water. This part of the cloak is protected by testace- 

 ous plates, which, by their union, form a lid to the 

 mouth of the shell for the protection of the contained 

 inhabitant. The valves of the lid are four in number, 

 two on each side the mesial line, or orifice. In some 

 genera the lateral valves are united. The operculum 

 of this order may with propriety be compared to the 

 shelly plates of the body of the preceding order and 

 the shelly body of this order corresponds to their pe- 

 duncle, circumstances indicated by the muscular at- 

 tachments of the animal. The continued action of the 

 valves of the lid, obviously assist in wearing down and 

 enlarging the aperture of the shell. 



There is little known, either with regard to the or- Molluscs, 

 gans of digestion or respiration, in the animals of this * -Y~~* 

 order. In their manner of reproduction, they appear 

 to resemble those of the preceding order. Ellis " found 

 the lower part of the shell, which contained a cavity 

 equal to two-thirds of the whole, full of spawn." The 

 genera of this order divide themselves into three fami- 

 lies, from circumstances connected with the shell. 



1. Family. The shell in this family consists of six 

 valves, and the lateral valves of the lid are divided. It 

 contains five genera. 



1. Tubicinella The form of the shell in this genus Tubicinel- 

 is inversely conical, the apex which constitutes the base la. 

 being truncated. It consists of a series of horizontal 



rings which mark the successive periods of growth, and 

 there are six vertical grooves, which indicate the divi- 

 sions of the valves. The increase of the shell, with 

 age, in this genus, probably takes place by the addition 

 of a new ring to the mouth. The testaceous plates of 

 the lid are all of equal size. The inferior aperture of 

 the shell is open, or simply closed by the integuments 

 of the cloak. The animal resides in the skin of the 

 whale, the lower rings being inserted in the fat, while 

 one or more of the upper ones appear above the cuticle. 

 The T. balcenarum is the only known species. 



In the remaining genera of this family, and the others 

 which follow, the shell is conical, its truncated apex 

 being the mouth ; and its mode of growth such as is 

 detailed in the general remarks on the order. 



2. Coronula. The base of the shell is open as in the Coronula. 

 preceding genus. The valves of the lid are unequal in 



size, the dorsal ones being small. The animals includ- 

 ed under this genus likewise inhabit the skin of the 

 whale. Several species of this genus are known. The 

 C. diadema holds a place in the 15ritish Fauna. 



3. Chelonobia In this genus the base of the shell is Chelono- 

 likewise open, but it differs from the preceding in the bia. 

 plates of the lid being all of equal size. The C. iestti- 

 dinaria, a species which resides on turtles, is the type 



of the genus. 



4*. Balanus. The shell in this genus is closed below BaUnus. 

 by a layer of shelly matter, which adheres to foreign 

 bodies, and conforms to the inequalities of their sur- 

 face. Nine species are described as natives of Bri- 

 tain. 



5. Acasta. In this genus the base of the shell is Acasta. 

 cup-shaped. The species reside in sponges, in the 

 substance of which the base and sides are imbedded. 

 One species, the Balitnus spongiostts of Montagu, is a 

 native of the English seas. 



2. Family. The valves of the shell in the genera of 

 this family are only four in number. 



1. Creusia. The base in this genus is funnel-shap- Creuaia. 

 ed. The lateral valves of the lid are united. The C. 

 spimtlosa of Dr. Leach is the type of the genus. It is 

 imbedded in the substance of Madrepores. 



2. Conia The shelly base in this genus conforms Conia. 



to the substance to which it is attached. The lateral 

 valves of the lid are separate. The C. porosa is the 

 type of the genus. 



3. Clisia. The base of the shell is spread on the ciiiia. 

 surface of the bodies to which it is attached. The la. 

 teral valves of the lid are united. The Balanus stra- 

 tus of British writers is the type of the genus. An im. 

 perfect representation of the animal is given by Cordi- 



ner, in his " Remarkable Ruins," table, Aggregate of 

 Corals. 



3. Family. The shell is undivided in the only genus 

 of this family which is known, termed Pyrgoma. The Pyrgomo. 



